Citrix and its virtualization partner Microsoft slammed rival VMware last year after rules changes led to the companies pulling out as sponsors of VMworld. Despite being hosted by VMware, rather than a neutral party, VMworld has become one of the most important conferences in the virtualization industry, where start-ups and established players alike showcase their latest technology.

But Citrix and Microsoft last year claimed VMware was trying to stifle the presence of competitors by instituting restrictive new rules on which companies can be sponsors and what types of technologies can be showcased. Although both Microsoft and Citrix sponsored VMworld in the past, they did not in 2009 because VMware started requiring sponsors to join VMware's official partner program.

That will change at VMworld in San Francisco, which takes place Aug. 30 to Sept. 2. Citrix has decided to become an official partner of VMware's and will be a "Silver" sponsor at VMworld. That's actually only the fifth highest level of VMworld sponsorship, but Citrix will have a greater presence at VMworld than Microsoft, which has still not joined VMware's partner program and will likely just have a small booth at the conference. VMworld's top sponsors are Cisco, Dell, EMC (which owns VMware) and NetApp.

"In order to participate more fully and openly at VMworld, we have joined the VMware partner program as a Technology Alliance Partner at the Select level," Citrix marketing official Kim Woodward wrote in a blog post about Citrix's change in strategy. "This has given us the opportunity to be a Silver sponsor of the event, and to showcase our technology and expand our booth size. We'll have double the space we did last year so more of you can come by without alarming the Fire Marshall."

One year after claiming VMware limits competition, Woodward did not take any direct shots at VMware but did say that "At Citrix, openness is engrained in both our culture and our technology."

At VMworld, Citrix will show demos of its desktop virtualization software running on a variety of devices, including the Apple iPad, which can run Windows 7 using Citrix Receiver.

Microsoft is listed near the bottom of Citrix's sponsors and exhibitors page, right after Metron-Athene Inc. and before Mitel.

But don't expect Microsoft to keep a low profile. In 2008 Microsoft handed out poker chips in a package that said "Looking for your best bet? You won't find it with VMware." In 2009 Microsoft spent the week complaining that VMware's rules prevented them from showcasing competing technology, even though VMware denied this rules interpretation. The rest of the exhibition hall was filled with technologies from other companies that compete against VMware.

While Microsoft still isn't a VMware partner, Microsoft has certified the VMware hypervisor to run with Windows, and Microsoft virtualization official Mike Neil promised in a recent interview to engage with customers at VMworld.

"From our perspective, regardless of whether customers are running on Hyper-V or VMware, first and foremost they're a Windows customer," Neil says.